ON THE ORIGIN OF VERTEBRATES. 



247 



may prove to be, we may, it seems to me, represent an ideal 

 compound retina of the crustacean type by combining the 

 investigations of Berger, Claus, Reichenbach, and Parker, in 

 the following figure. 



The comparison of this figure (fig. 6) with that of the Ammo- 

 coetes retina (fig. 4) shows how great is the similarity of the latter 

 with the Arthropod type, and how the very points in which it 



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f 2 



; 



Fig. 6. — Ideal diagram of the layers in a Crustacean eye. The retina is divided 

 into an epithelial part C (the layer of retinular cells and rhabdomes), and 

 a neurodermal or cerebral part, which is formed of A, the ganglion of the 

 optic nerve, and B, the ganglion of the retina. 1, optic nerve iibres which 

 cross at their entrance into the retina ; 2, int. molecular layer with its two 

 rows of cells ; 3, int. nuclear layer ; 4, Reichenbach's double row of large 

 lightly-staining cells ; 5, layer of terminal retinal fibres ; 6, ext. nuclear 

 layer (nuclei of retinular cells as seen in fig. 3) ; 7, ext. limiting membrane ; 

 8, layer of crystalline cones ; 9, cornea. 



deviates from the recognised vertebrate type are explainable 

 by comparison with that of the Arthropod. The most striking 

 difference between the retinas in the two figures is that the 

 layer of terminal nerve fibres (5, fig. 6), which, after all, are 

 only the elongated terminations of the retinal cells belonging to 

 Parker's neurons of the first order, is very much longer than in 



